It is known that sonars are conventionally divided into two major categories. One of these categories comprises active sonars in which an acoustic signal is emitted within the sea and the echoes returned by the obstacles whose position one wishes to ascertain are gathered. The other category comprises passive sonars which are concerned merely with listening for the noise radiated within the sea by various sources, for example the propellers of a boat. Such is the case in particular for linear acoustic arrays which are towed behind the boat and which stretch over distances of several hundred meters.
This technique has been employed to design devices which enable these arrays, essentially characteristic of a passive sonar, to be made to play a role of active sonar, by using a distinct special-purpose array to emit an acoustic signal whose echoes are received by these arrays. This function is termed the "activation function".
Referring to FIG. 1, the linear acoustic array 101 is towed by a boat 102, and in order to use it in an active manner this same boat 102 tows a fish 103 which comprises an acoustic emitter. Given the ranges of frequencies used by a linear acoustic array, which lie within the low-frequency range, the emitter contained in the fish emits in a near-omnidirectional manner by virtue of the small size of its emission arrays relative to the emission wavelength .lambda., made necessary by the dimensions of the fish. Such a device is in particular described in French Patent Application No. 91 03853 filed by the applicant on 29 Mar. 1991 and published on 2 Oct. 1992 under number 2 674 717.
There is also a type of active sonar, known as a "dipping" sonar and represented in FIG. 2, which includes receiving arrays 201 carried by folding arms 211 fixed to a submersible buoy body 221 and which are deployed when the body of the buoy is submerged at the required depth. This body is suspended by a cable wound on a winch fixed to a helicopter 202. It also includes an emitter 203 situated for example at the base of the buoy. The helicopter, which hovers at a fixed spot, unwinds the cable, and the body which carries the receiving array is submerged in the water. Such a sonar is in particular described in French Patent Application No. 86 13485 filed by the applicant on 26 Sep. 1986 and published on 1 Apr. 1988 under number 2 604 530. Although the frequencies used are often higher than the frequencies used in the linear array of FIG. 1, the small size of the body of the buoy precludes the fixing of a very large emitter and so the same relative conditions as before again obtain, involving substantially omnidirectional radiation of the emitter.
As a consequence of the omnidirectional nature of the radiation, these devices have the drawback of limiting the power emitted, so as not to run up against the phenomenon of cavitation, this limiting the detection range since the sound level emitted under these conditions is relatively low and since the directivity index cannot be altered precisely because of this omnidirectional nature.
Moreover, since we are at low frequency and because this antenna is relatively small, nuisance acoustic interactions are obtained between the various transducers, making control of the directivity patterns more difficult.
Finally, and more particularly in the case of a linear acoustic array, the handling of a fish, even if it is relatively small in size, involves implementational constraints related to weight and bulk, which are highly constraining during placement into the water and recovery, and hence entailing a very high cost.